Magnetron sputtering involves the arrangement of permanent or electromagnets behind a target material (cathode), and applying a magnetic field to the target. The applied magnetic field transmits through the target and focuses a discharge plasma onto the front of the target. The target front surface is atomized with subsequent deposition of the target atoms on top of an evolving thin film device positioned adjacent to the target.
Magnetron sputtering of magnetic target materials is very prevalent in the electronics industry, particularly in the fabrication of semiconductor and data storage devices. Due to the soft magnetic nature of magnetic target alloys, there is considerable shunting of the applied magnetic field in the bulk of the target. This in turn results in reduced target utilization due to focussing of the transmitted magnetic field in the erosion groove formed as a result of the shunting. This focussing effect is exacerbated with increasing material permeability (which corresponds to decreasing material PTF).
It is well known that reducing target material permeability promotes a less severe erosion profile which enhances target material utilization and subsequently contributes to a reduction in material cost. The presence of severe target erosion profiles also promotes a point source sputtering phenomena which can result in less than optimum deposited film thickness uniformity. Therefore, decreasing target material permeability has the added benefit of increasing deposited film thickness uniformity.
The PTF of a magnetic target is defined as the ratio of transmitted magnetic field to applied magnetic field. A PTF value of 100% is indicative of a non-magnetic material where none of the applied field is shunted through the bulk of the target. The PTF of magnetic target materials is typically specified in the range of 0 to 100%, with the majority of commercially produced materials exhibiting values between 10 to 95%.
There are several different techniques for measuring product PTF. One technique involves placing a 4.4 (+/-0.4) kilogauss bar magnet in contact on one side of the target material and monitoring the transmitted field using a axial Hall probe in contact on the other side of the target material. The maximum value of the magnetic field transmitted through the bulk of the target divided by the applied field strength in the absence of the target between the magnet and probe (maintained at the same distance apart as when the target was between them) is defined as the PTF. PTF can be expressed as either a fraction or a percent.
Another technique for measuring PTF involves using a horseshoe magnet and a transverse Hall probe. The PTF values measured using different magnet and probe arrangements are found to exhibit good linear correlation for the values of magnet field strength typically utilized in the industry. The PTF measurement techniques are constructed to realistically approximate the applied magnetic flux occurring in an actual magnetron sputtering machine. Therefore, PTF measurements have direct applicability to a target material's performance during magnetron sputtering. FIG. 1 depicts the bar magnet and axial Hall probe contact PTF measurement set-up utilized for the measurements discussed hereafter.
Magnetic material PTF and permeability are not mutually exclusive. Rather, there is a very strong inverse correlation between PTF and maximum permeability of magnetic materials. Values of material magnetic permeability can be very precisely determined by using vibrating-sample-magnetometer (VSM) techniques in accordance with ASTM Standard A 894-89. Descriptions of sample geometry and calculation of the appropriate demagnetization factors for permeability determination are well known in the art. See, for example, Bozarth, Ferromagnetism, p. 846.
Magnetic target PTF is a strong function of both target chemistry and the thermomechanical techniques utilized during target fabrication. For alloys that do not possess inherently high PTF as a result of their stoichiometry (PTF&lt;85%), it is possible to increase product PTF by various thermomechanical manipulations during product fabrication.
Typical fabrication of Ni, Co and Co-alloy targets involves casting, hot-rolling and either heat treatment or cold-rolling or a combination of heat treatment followed by cold-rolling. It is known, for example, that heat treating and cold-rolling of magnetic target materials can increase product PTF. Heat treatment of Co--Cr--Ta--(Pt) alloys below 2200.degree. F. has been shown to increase the PTF by promoting matrix crystallographic phase transformation from FCC (face centered cubic) to HCP (hexagonal close packed). The driving force for this martensitic transformation is provided by the interfacial strain associated with the precipitation of Co--Ta semi-coherent precipitates during heat treatment. Chan et al., Magnetism and Magnetic Materials, vol. 79, pp 95-108 (1989), suggests that the greater mobility of domain walls in the HCP phase compared with the FCC phase in Co--Cr base alloys contributes to the increase in target PTF with microstructural phase transformation from FCC to HCP.
It is suggested in Weigert et al., Mat. Sci. and Eng., A 139, pp 359-363 (1991), that cold-rolling of (62 to 80 atomic %) Co-(18 to 30 atomic %) Ni-(O to 8 atomic %) Cr alloys immediately after the hot-rolling process results in an increase in product PTF. This suggests that the increase in PTF is a result of the cold-deformation induced [0001] basal hexagonal texture ([0001] hexagonal directions aligned perpendicular to the target surface). A similar result is disclosed in Uchida et al., U.S. Pat. No. 5,468,305 for Co-(O. 1 to 40 atomic %) Ni-(O.1 to 40 atomic %) Pt-(4 to 25 atomic %) Cr alloys cold-rolled by not more than a 10% total reduction after the hot-rolling process. Uchida et al. claim that the cold-deformation induces internal strain in the alloy which reduces magnetic permeability. As mentioned earlier, a reduction in magnetic permeability corresponds to an increase in product PTF.
In summary, the prior art teaches cold-rolling as a means of increasing product PTF by either enhancing the basal texture component of the HCP phase or increasing the overall alloy internal strain density. It is possible that both the texture and strain mechanisms promote an overall increase alloy PTF.
Three issues are specifically not addressed in the prior art: (1) The utilization of warm-rolling practices to enhance product PTF, (2) The very pronounced effect of directionality during hot and cold-rolling on product PTF and (3) the further enhancement of target material PTF by employing post warm-rolling heat treatment practices.
Current data storage technology utilizes a myriad of multi-component multi-phasic alloys that tend to be very hard and brittle. Adverse effects associated with cold-rolling of these alloys include the following: (1) severe deformation results in a high risk of plate cracking, warping and chipping; (2) large residual stresses result in significant difficulties during final product machining; (3) a substantial amount of wear and damage to the rolling mills typically used to process these materials; and (4) due to the severity of the cold-rolling process, the overall reduction is commonly not enough to guarantee uniform strain and texture gradients throughout the thickness of the part.
The presence of microstructural gradients in the part can be deleterious to product consistency during final sputtering application which involves the successive atomic removal of material from the target surface. The combination of these factors results in high product cost and less than optimum performance consistency.
Thus, despite the advantages of using cold-rolling for increasing PTF, there remains a need in the art for an improved process which further increases pass-through-flux and eliminates the problems associated with cold-rolling.